Flat field grating spectrometers find wide application in many analytical instruments, such as spectrophotometers and colorimeters, used to practice spectroscopy in the ultraviolet, visible, near-infrared, and mid-infrared regions of the electromagnetic spectrum. A flat field, i.e., spectral image plane, is desirable when the detection means is flat, such as with diode array detectors, image intensifier tubes, or photographic plates. With development of aberration-corrected, concave holographic gratings, it became possible to design flat field spectrometer optical systems using the holographic grating as the only optical element between the entrance slit and the detector.
However to obtain such a flat field concave holographic grating, the usable image plane is tilted and displaced far from its preferred Rowland circle location. As a result, the linear dispersion varies a large amount from one end of the field to the other. This is a particular disadvantage for diode array detectors, which usually have equally spaced elements that then produce unequal spectral resolution across the field. The axis of the cone of energy incident at the detector is far from normal to the detector, complicating the design of order sorting filters.
Diode-array, concave holographic grating spectrometers have been made in which the entrance slit lies below the center of the spectral image plane, which is displaced above the plane containing the normal to the grating by the same distance as the entrance slit is below this plane. This design has two significant disadvantages. First, the spectral focus lies on a curved image surface, which limits the spectral range over which acceptable resolution can be obtained, and the out-of-plane design results in larger aberrations than those of an in-plane design. This factor limits the spectral resolution obtainable, and, to a considerable extent, the throughput since the entrance slit height must be restricted to obtain reasonable resolution.
Other prior designs place the entrance slit and detector in-plane, with the slit located beyond the end of the spectral image plane. The holographic grating parameters are adjusted to provide minimal aberrations, including astigmatism, over a flat spectral field. These designs have good spectral resolution over only a limited spectral range.
To overcome problems of spectral resolution, prior spectrometers have often reduced the entendu (optical throughput) (cm.sup.2 -ster) by reducing the numerical aperture of the grating, the area of the entrance slit and detector element, or both. The resultant loss of energy reduces the signal-to-noise ratio obtainable in measuring spectroscopic data.
Prior flat field concave holographic grating instruments have used a single order of the grating, further limiting the usable spectral range. In many applications it is desirable to obtain simultaneous measurements extending over two spectral regions, e.g., the ultraviolet and visible or the visible and the near-infrared, which span a spectral range exceeding a two to one ratio of maximum to minimum wavelengths. In many cases, different detector arrays may be required for different spectral regions. Combining two different detector materials in a single array results in a gap between the two subarrays.